An AM transmitter using only one tube? Yes, and a very commonly available one
at that. This circuit is a bare-bones transmitter that, with a 10-foot (3 meter)
run of wire as an antenna, will readily transmit an AM signal to nearby radios.
What's more, it doubles as an accurate and convenient 1 MHz marker generator for
aligning shortwave radios!

The classic 6BE6 tube (and its 12 volt variant the 12BE6) was extremely common
throughout the vacuum tube's latter "glory days". It was based on the
earlier octal 6SA7/12SA7, which was functionally equivalent. Its purpose in life
was to be the local oscillator and mixer in AM radios, and a kazillion of them
were made to fill that purpose. But that's not all it can do! With only a few
external parts and suitable power supplies, it forms a complete stand-alone
AM transmitter!

The first circuit is the
"Muntz" version of
a common one-tube transmitter using a 12BE6 pentagrid converter (a.k.a. heptode).
This marvel of engineering is at the same time an oscillator, mixer,
and amplifier -- all in one valve structure.

Since its intended purpose was as a converter for superheterodyne receivers,
its output power capability is quite low; in practise, about 3 milliwatts
(into a 50 ohm load). With a short "wet noodle" antenna, the actual
radiated power will be a great deal less. It's enough, however, to give good
reception within a radius of about 30' (10 meters).

This is one case where the practise of "tube rolling" (trying
different samples of the same type) really pays off. If you have an
oscilloscope, you can monitor the RF output while changing the level of
a constant audio signal. There will be significant differences, and
a good 12BE6 works amazingly well, achieving up to about 80% modulation.
(Keep in mind that the difference between 90% and 100% modulation is
less than one dB.)

Robert Casey
has done excellent work capturing
'scope traces using the 6CS6 pentagrid
converter. His conclusion is that these are superior to the ordinary run
of 6BE6/12BE6s. However, since I had about a dozen of the 'BE6s, I was able to
cull the best of them and achieve similar results.

There's a certain nostalgia to these circuits; I used to build them as a
teenager, but never with crystal control. They were all along the lines of
the "Gypsy" variant, using a variable capacitor
and inductor to tune the oscillator. The modified Colpitts scheme shown here
worked right away, and is stable and robust.

The circuit around the first grid (pin 1) and cathode (pin 2) forms the
basis of a crystal-controlled Colpitts oscillator. Feedback is from the
cathode circuit to the grid circuit via the capacitive divider C2 and C3.
C3 is variable, to allow exact tuning to 1.00 MHz. (Needless to say, you'll
have to change the crystal frequency if you have a station at 1000 kHz in your
area.)

The crystal may be the most challenging item to find. If all else fails, you
can get it through any
NTEdistributor as NTE 650.
(Not exactly cheap, though; if you poke around, you can probably find cheaper
sources.)

Grid 2 acts as a "virtual anode" for the oscillator. Note that it's
essentially at RF ground, shielding the remaining grids and the plate circuit from
the oscillator portion. Similarly, there's another grid (G4) tied to the same
point, isolating G3 from the rest of the tube.

G3 is our audio input. The signal applied here mixes with the oscillator current,
causing an amplitude-modulated RF signal to appear at the plate. A tuned circuit
in the plate is used to peak response, maximizing output and helping to suppress
harmonics.

However, with only one tuned circuit, there is still plenty of harmonic content.
The second and third harmonics are down only about 20 dB, with higher harmonics
dropping off. This suggests an excellent alternate usage for this little
transmitter: a marker generator for aligning shortwave receivers! Its signal will
appear at 1 MHz, 2 MHz, 3 MHz, etc. up to a practical maximum of about 15 MHz.

Tuning the circuit is very simple, and can be done by ear if necessary. Connect
your antenna and audio input source, and monitor the signal on a nearby AM
radio. Tune the one adjustable capacitor for maximum signal, and adjust your
audio level for a clear, loud signal without distortion. If you have an RF probe
or oscilloscope, you can use them for a more exact tuning adjustment.

I don't have an actual photo of the Muntz, since it was later morphed into the
Bean Counter variation, covered next.